Fate of separate chiral transitions at finite $\mu_I$ under the influence of mismatched vector interactions

Abstract

The flavor-mixing induced by the mismatched vector-isoscalar and vector-isovector interactions at finite baryon chemical potential $\mu$ and isospin chemical potential $\mu_I$ is demonstrated in the Nambu-Jona-Lasinio (NJL) type model of QCD. The influence of this non-anomaly flavor-mixing on the possible separate chiral transitions at nonzero $\mu_I$ is studied under the assumption of the effective restoration of the $U(1)_A$ symmetry. We find that for the weak isospin asymmetry, the two separate phase boundaries found previously can be converted into one only if the vector-isovector coupling $g_v^v$ is significantly stronger than the vector-isoscalar one $g_v^s$ without the axial anomaly. When the weak Kabayashi-Maskawa-'t Hooft (KMT) interaction is included, we find that the separation of the chiral transition with two critical endpoints for the relatively strong isospin asymmetry can still be removed owning to the vector interactions. In this case, it is not the vector coupling difference but the strength of $g_v^v$ which is crucial for the only phase boundary. We also point out that, in the NJL-type model with mismatched vector interactions, the recently proposed equivalence for chiral transitions at finite $\mu$ and $\mu_I$ does not hold even at the mean field approximation.